Seismic surveying for oil, gas and other minerals is a well established practice. According to this method, mechanical vibrations are imparted to the earth by means of, e.g., dropping a heavy weight, detonating a charge of explosives or by shaking a heavy object. The acoustic wave thus generated travels downwardly through the earth, is reflected from interfaces between layers of various types of rock and returns upwardly to the surface of the earth at which point it may be detected by one or more geophones. Typically, a large number of geophones are arrayed in a "spread" on the surface of the earth. By comparison of the times taken by acoustic waves generated and detected at differing points on the earth's surface, a representation of the relative shape of the subterranean interfaces can be generated which is of use to geophysicists in determining the likely presence of oil, gas and other minerals.
One of the chief problems in this seismic surveying technique is that of achieving an adequate signal-to-noise ratio in the geophone output, so as to be able to process the signals thus referred to yield meaningful output. As the acoustic waves imparted to the earth must travel great distances downwardly into the earth, be reflected, and return upwardly (typically the interfaces which are to be mapped are on the order of 15,000 feet below the earth's surface) the attenuation of the signal alone causes relatively low signal-to-noise ratios. In addition, noise is always present in the earth's crust of frequencies comparable to that of the acoustic energy imparted to the earth. Moreover, frequently the waves from a given source can travel in more than one path and at differing velocities to reach the same geophones, thus further obscuring the data and confusing the representation of the subterranean structure generated.
One of the more common methods of reducing noise in seismic records is so-called common depth point (CDP) surveying. According to this method, a plurality of signals are recorded with respect to sources and detectors disposed on either side of a central point at differing distances therefrom such that it can be assumed that the acoustic records recorded are of reflections from a single point on a given layer of subterranean rock, i.e., a common depth point. When these records are mathematically or electrically summed, the noise, being random, tends to cancel out while the signal is reinforced, thus yielding an improvement in the signal-to-noise ratio. It is usual to vary the distance between source and detector, or "offset" between zero and on the order of 10-12,000 feet or more, surveying continuously so as to provide a large number of records for such CDP surveying for summation of records of acoustic energy reflected from a common depth point deep within the earth's crust. However, there are areas of the earth where subterranean conditions render this CDP method alone insufficient to generate a record of exploration having a sufficiently high signal-to-noise ratio as to allow a good representation of the subterranean structure of the earth to be generated. One structure which is known to be very difficult to survey using the CDP method comprises successive layers of evaporites of various types disposed within the earth. It appears that the interfaces between such layers are such as to cause a substantial attenuation of the acoustic waves thus reducing the signal strength which is received at the geophones and lowering the signal-to-noise ratio.
Accordingly, it has been and remains a need of the art to provide an improved signal-to-noise ratio for seismic exploration for oil, gas and other minerals.